In order to detect abnormalities, faults, and so forth of optical transmission devices, optical transmitters and optical receivers, optical transmission paths, and so forth, devices that monitor signal quality (for example, an optical-signal-to-noise ratio (OSNR)) of optical signals being used are used in some optical transmission networks (or systems).
In next-generation optical networks, it is expected that the wavelengths and routes of optical signals will be more dynamically changed. It is therefore considered that devices that monitor signal quality will become more important in the future.
As exemplary techniques for estimating or monitoring signal quality of optical signals, the techniques disclosed in U.S. Patent Application Publication No. 2012/0106951, U.S. Pat. No. 7,440,170, Japanese Laid-Open Patent Publication No. 2010-226499, and Japanese Laid-Open Patent Publication No. 2008-085836 are known.
In the technique described in U.S. Patent Application Publication No. 2012/0106951, as illustrated in FIG. 16, amplitude-modulated data (hereinafter referred to as “AM data”) 402 is superimposed on a transmission-light (optical) main signal (optical payload) 400. Note that FIG. 16 illustrates changes of optical power over time, and reference numeral 401 denotes average power of the optical main signal 400.
The optical main signal on which the AM data is superimposed is filtered around the signal center wavelength (filter position 1) and at a wavelength (filter position 2) shifted from the signal center wavelength by using a narrow-band optical filter as illustrated in FIG. 17. Photoelectric conversion of the filtered optical signal is performed, and the direct-current (DC) component and the alternating current (AC) component of the signal after the photoelectric conversion are measured.
In other words, VADC-DC1 and VADC-AC1 are measured as the DC component and the AC component at the filter position 1, respectively, and VADC-DC2 and VADC-AC2 are measured as the DC component and the AC component at the filter position 2, respectively.
Here, VADC-DC1, VADC-AC1, VADC-DC2, and VADC-AC2 represent voltage values obtained by analog-to-digital conversion (ADC) of the signals after photoelectric conversion, and are expressed, as illustrated in FIG. 17, by the following formulas (1. 1) to (1. 4).VADC-DC1=VSig-DC+VASE  (1. 1)VADC-AC1=VSig-AC  (1. 2)VADC-DC2=VSig-DC*R+VASE  (1. 3)VADC-AC2=VSig-AC*R  (1. 4)
Note that “VSig-DC” and “VSig-Ac” represent the voltages of the DC component and the AC component of the main signal, respectively, and “VASE” represents the voltage of a spontaneous emission light component. “R” represents a damping coefficient related to shifting from the filter position 1 to the filter position 2, and is a value satisfying the condition of 0<R<1.
The OSNR to be determined is a ratio (PSig/PASE) of main signal optical power (PSig) to spontaneous emission optical power (PASE) and therefore be determined by (VSig-DC/VASE) as expressed in the following formula (1. 5).OSNR=PSig/PASE=VSig-DC/VASE  (1. 5)
Here, since four equations (1. 1) to (1. 4) exist for four unknowns in the aforementioned formulas (1. 1) to (1. 4), VSig-DC/VASE may be determined. In this way, in the technique disclosed in U.S. Patent Application Publication No. 2012/0106951, the OSNR is estimated from the ratio between the DC component and the AC component of a signal obtained by filtering an optical main signal on which AM data is superimposed, at different filter positions 1 and 2.
Next, U.S. Pat. No. 7,440,170 describes that the OSNR is estimated from power of an optical signal that has been caused to pass through a Mach-Zehnder interference (MZI) filter. That is, the transmission characteristic of an MZI filter, which has a periodic transmission characteristic, is shifted through temperature control, and transmitted optical power (the maximum (PMAX) and the minimum (PMIN) of each of the signal light component and the noise light component) is measured.
Here, assuming that PMAX/PMIN=R and that the noise equivalent bandwidth is represented by NEB (constant), the OSNR to be determined is expressed by the following formula (2. 1). Ps/Pn in formula (2. 1) is expressed by the following formula (2. 2).
                              O          ⁢                                          ⁢          S          ⁢                                          ⁢          N          ⁢                                          ⁢          R                =                  10          ⁢          log          ⁢                                          ⁢          10          ⁢                      (                                                            P                  s                                                  P                  n                                            ·                                                N                  ⁢                                                                          ⁢                  E                  ⁢                                                                          ⁢                  B                                                  12.5                  ⁢                                                                          ⁢                  GHz                                                      )                                              (        2.1        )                                                      P            s                                P            n                          =                              [                                                                                (                                          n                      +                      1                                        )                                    ⁢                                      (                                          s                      -                      n                                        )                                                                                        (                                          R                      -                      n                                        )                                    ⁢                                      (                                          s                      +                      1                                        )                                                              -                                                n                  +                  1                                                  s                  +                  1                                                      ]                                -            1                                              (        2.2        )            
In formula (2. 1), s represents the ratio between the maximum and the minimum of transmitted optical power from the MZI filter for signal light that does not contain a noise light component, and n represents the ratio between the maximum and the minimum of transmitted optical power of the MZI filter for the noise light component that does not contain a signal light component. The values of s and n are values that may be calculated from the characteristic of the MZI filter. Therefore, from the measured value R and the values of s and n, the OSNR may be determined by the above formula (2. 1) and formula (2. 2).
Next, in the technique described in Japanese Laid-Open Patent Publication No. 2010-226499, in a transmission device, the frequencies of carrier light are modulated at frequencies that are different for respective polarization channels, polarization-multiplexing is performed, and the result is transmitted to an optical transmission path. In an optical signal quality monitoring device, four types of polarization components are detected for each light intensity component for every unit optical frequency of a data-modulated optical signal received from the optical transmission path, and a Stokes vector is calculated by computing the intensity of each type of polarization component. Then, the intensity of a frequency-modulated component added to each polarization channel is extracted from the calculated Stokes vector. Thus, the polarization state for every polarization channel may be monitored without optical polarization splitting of data-modulated light.
Next, Japanese Laid-Open Patent Publication No. 2008-085836 describes an optical signal quality monitoring device in which an input optical signal and a local oscillator signal are mixed, at least one beat component of the mixed signal is extracted by a band-pass filter, and the intensity of the extracted beat component is detected. With this optical signal quality monitoring device, if the quality (OSNR) of the input light signal is good, the beat components do not spread by the mixing. When the mixed signal is caused to pass through the band-pass filter, most of the beat components pass through the filter without being filtered out. If, however, the OSNR of the input light signal is poor, the beat components spread by the mixing. When the mixed signal is caused to pass through the band-pass filter, some of the beat components are filtered out. In this way, the OSNR of an input light signal may be monitored by utilizing the fact that the power of an output signal of the band-pass filter varies depending on whether the OSNR of the input light signal is good or poor.
However, in the technique described in U.S. Patent Application Publication No. 2012/0106951, since the AM data is superimposed on the main signal, degradation (penalty) occurs in the main signal. Additionally, in the technique described in U.S. Pat. No. 7,440,170, if band narrowing arises from the fact that an optical signal passes through a plurality of wavelength selection switches (WSS) for transmission, the value of PMAX/PMIN=R described above changes. This results in an OSNR calculation error. Additionally, the MZI filter has to be temperature controlled, and therefore control and the configuration are made complicated.
Note that the technique described in Japanese Laid-Open Patent Publication No. 2010-226499 is capable of monitoring the polarization state, but is not capable of monitoring the OSNR. Additionally, the technique described in Japanese Laid-Open Patent Publication No. 2008-085836 uses a local oscillator signal source, and therefore the configuration becomes complicated.